A. P. Walsh

University College London, Londinium, England, United Kingdom

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Publications (57)97.41 Total impact

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    ABSTRACT: We present a detailed ground-based and in-situ examination of two substorms on 9th April 2011 providing one of the most detailed observations of a series of activations to date. Using auroral observations we demonstrate that the initial signature of substorm onset is a localised brightening on closed field lines. In both cases auroral onset precedes any geosynchronous indications of substorm onset. Despite excellent in-situ coverage of the near-midnight plasmasheet there is no evidence of tail flows or topological changes which might indicate the initiation of reconnection and related disturbances prior to auroral onset. Similarly, no auroral streamers are observed prior to either onset. For the second substorm, following auroral onset and dipolarisation of the geosynchronous field there is evidence of a rapid thinning of the plasma sheet followed by a secondary auroral activation and dipolarisation of the magnetotail outside of geosynchronous orbit. This morphological change suggests that NENL reconnection is eventually triggered in the tail, but only following the earlier auroral onset. These observations demonstrate that expansion phase onset is not always initiated by reconnection in the tail. Finally, our observations suggest that activations at the near-Earth neutral line, or related to plasma instabilities and near-Earth onset, can develop independently during expansion phase onset. In this paradigm, free energy stored during the growth phase can be stored in the stretched tail and the inner magnetosphere where the dipole field begins to stretch. During expansion phase onset either region could become unstable, independent of the other, leading to substorm initiation and onset.
    Journal of Geophysical Research: Space Physics 10/2014; 119(12). DOI:10.1002/2014JA019795 · 3.44 Impact Factor
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    ABSTRACT: In this paper we utilize nine years of Cluster observations in the Earth's magnetotail to investigate electron pitch-angle/energy distributions. We mainly concentrate on the population of anisotropic electrons with a dominance of the parallel phase space density and energies larger than 100 eV. The energy distribution of this population strongly varies with the downtail distance (the near-Earth x > − 12RE and the tail x ∈ [−20, − 12]RE) and along the dawn-dusk direction (the midnight |y| < 10RE and the flanks |y| > 10RE). In the tail-midnight domain the electron anisotropic population is present at all energy ranges up to 20 keV, while at the tail-flank domain only the energy range below several keV is filled by this population. In the near-Earth domain the only anisotropic electrons are cold, with energies less than 1 keV. We investigate the dependence of the energy distribution of the anisotropic electron population on the system parameters for the midnight-tail domain where the main statistics is collected. The increase of the Bz GSM component of the magnetic field corresponds to an increasing energy range filled by anisotropic electrons. The increase of the electron temperature anisotropy is providedby the enhancement of the anisotropic population at all energies up to 20 keV. There is no distinct dependence of the electron anisotropic population on the plasma flow velocity. The increase of the electron temperature corresponds to shifting of the electron anisotropic population towards higher energies. We propose a simple model of the pitch-angle/energy distribution of this electron population and discuss the origin of anisotropic electrons in the magnetotail.
    Journal of Geophysical Research: Space Physics 09/2014; 119(9). DOI:10.1002/2014JA020350 · 3.44 Impact Factor
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    ABSTRACT: Spacecraft observations have established that all known planets with an internal magnetic field, as part of their interaction with the solar wind, possess well-developed magnetic tails, stretching vast distances on the nightside of the planets. In this review paper we focus on the magnetotails of Mercury, Earth, Jupiter and Saturn, four planets which possess well-developed tails and which have been visited by several spacecraft over the years. The fundamental physical processes of reconnection, convection, and charged particle acceleration are common to the magnetic tails of Mercury, Earth, Jupiter and Saturn. The great differences in solar wind conditions, planetary rotation rates, internal plasma sources, ionospheric properties, and physical dimensions from Mercury's small magnetosphere to the giant magnetospheres of Jupiter and Saturn provide an outstanding opportunity to extend our understanding of the influence of such factors on basic processes. In this review article, we study the four planetary environments of Mercury, Earth, Jupiter and Saturn, comparing their common features and contrasting their unique dynamics.
    Space Science Reviews 08/2014; 182(1-4):85-154. DOI:10.1007/s11214-014-0060-8 · 5.87 Impact Factor
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    ABSTRACT: Flux transfer events (FTEs) are signatures of transient reconnection at the dayside magnetopause, transporting flux from the dayside of the magnetosphere into the magnetotail lobes. They have previously been observed to contain a combination of magnetosheath and magnetospheric plasma. On 12 February 2007, the four Cluster spacecraft were widely separated across the magnetopause and observed a crater-like FTE as they crossed the Earth's dayside magnetopause through its low-latitude boundary layer. The particle instruments on the Cluster spacecraft were in burst mode and returning data providing 3-D velocity distribution functions (VDFs) at 4 s resolution during the observation of this FTE. Moreover, the magnetic field observed during the event remained closely aligned with the spacecraft spin axis and thus we have been able to use these 3-D data to reconstruct nearly full pitch angle distributions of electrons and ions at high time resolution (up to 32 times faster than available from the normal mode data stream). These observations within the boundary layer and inside the core of the FTE show that both the interior and the surrounding structure of the FTE consist of multiple individual layers of plasma, in greater number than previously identified. Our observations show a cold plasma inside the core, a thin layer of antiparallel-moving electrons at the edge of FTE itself, and field-aligned ions with Alfvenic speeds at the trailing edge of the FTE. We discuss the plasma characteristics in these FTE layers, their possible relevance to the magnetopause reconnection processes and attempt to distinguish which of the various different FTE models may be relevant in this case. These data are particularly relevant given the impending launch of NASA's MMS mission, for which similar observations are expected to be more routine.
    Annales Geophysicae 01/2014; 32(9-9):1093-1117. DOI:10.5194/angeo-32-1093-2014 · 1.68 Impact Factor
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    ABSTRACT: 2012 Rishbeth prizewinners C Forsyth, A N Fazakerley, A P Walsh and C J Owen address the auroral acceleration region.
    Astronomy & Geophysics 12/2013; 54(6). DOI:10.1093/astrogeo/att209 · 0.34 Impact Factor
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    ABSTRACT: [1] We survey the properties of electron pitch angle distributions in the magnetotail plasma sheet at a distance between 15 and 19 RE from the Earth, using data from the Plasma Electron and Current Experiment (PEACE) instrument. We limit our survey to those pitch angle distributions measured when the interplanetary magnetic field (IMF) had been steadily northward or steadily southward for the previous 3 h. We find that, at sub-keV energies, the plasma sheet electron pitch angle distribution has an anisotropy such that there is a higher differential energy flux of electrons in the (anti-) field-aligned directions. Fitting the measured pitch angle distributions with both a single and two component kappa distribution reveals that this anisotropy is the result of the presence of a second, cold, component of electrons that is observed more often than not, and occurs during both the northward and southward IMF intervals. We present evidence that suggests the cold electron component has an ionospheric, rather than magnetosheath, source and is linked to the large-scale field-aligned current systems that couple the magnetosphere and ionosphere.
    Journal of Geophysical Research: Space Physics 10/2013; 118(10). DOI:10.1002/jgra.50553 · 3.44 Impact Factor
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    ABSTRACT: The solar wind electron distribution is observed near and within 1 AU to consist of three components: a thermal core, a suprathermal halo, and a suprathermal strahl. The former two components are isotropic, while the strahl is field aligned and flows outward along the interplanetary magnetic field. The evolution of solar wind electrons with heliocentric distance is poorly understood; although the halo is thought to be formed through pitch angle (PA) scattering of the strahl, the responsible physical process has not been conclusively identified. Measurements of solar wind electrons throughout the heliosphere are required to solve this problem. We present the first observations of the suprathermal components of the solar wind electron distribution made outside 5 AU. We find indications of a strahl component narrower than that predicted by extrapolating observations and models of electrons in the inner heliosphere, suggesting the rate of electron pitch angle scattering in the solar wind can decrease with increasing heliocentric distance.
    Geophysical Research Letters 06/2013; 40(11):2495-2499. DOI:10.1002/grl.50529 · 4.46 Impact Factor
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    ABSTRACT: A comparison of magnetotail flapping (the upand-down wavy motion) between the Earth and the two giant planets Jupiter and Saturn has been performed through investigation of the current sheet normal of the magnetotail. Magnetotail flapping is commonly observed in the Earth’s magnetotail. Due to single spacecraft missions at the giant planets, the normal is determined through minimum variance analysis of magnetometer data during multiple intervals when the spacecraft crossed through the current sheet. It is shown that indeed a case can be made that magnetotail flapping also occurs at Jupiter and Saturn. Calculations of the wave period using generic magnetotail models show that the observed periods are much shorter than their theoretical estimates, and that this discrepancy can be caused by unknown input parameters for the tail models (e.g., current sheet thickness) and by possible Doppler shifting of the waves in the spacecraft frame through the fast rotation of the giant planets.
    Annales Geophysicae 05/2013; 31:817. DOI:10.5194/angeo-31-817-2013 · 1.68 Impact Factor
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    ABSTRACT: Since launch in 2000, the four ESA Cluster spacecraft have each crossed the dayside magnetopause region thousands of times. Many previous studies presenting analysis of data from the mission, have contributed to a better understanding of the structure and dynamics of that interface and its associated boundary layers. While 2D electron pitch angle distributions (PAD) are routinely produced by the PEACE sensors on Cluster at spacecraft spin resolution (4s), the structures in this region are known to undergo changes on faster timescales than this, in response to both external drivers and internal dynamic processes. However, in certain circumstances, near-complete pitch angle distributions can be obtained at higher time resolution using Cluster burst mode data, facilitating a more detailed analysis of the particle behaviour near the magnetopause. In this paper we present an event during which the four spacecraft made outbound crossings through the low latitude boundary layer while the magnetic field orientation allowed a full pitch angle distribution of electrons to be constructed (every 1/8 s). The four Cluster spacecraft were in the 'multi-scale' formation with separations between individual pairs of spacecraft of either ~8000 or ~800 km. During the event in question, the Cluster spacecraft observed two flux transfer events (FTEs) and made a rapid (~16s) crossing of the magnetopause. The first FTE was most prominent in the C1 data a few minutes before the spacecraft crossed the magnetopause; and the second FTE was observed by C2 just before its magnetopause crossing. Additionally, C1 detected the signature associated with the second FTE in the magnetosheath, and the data from C3 show a disturbance in the low latitude boundary layer that also appears to be related to this FTE. We have utilized the high time resolution pitch angle distributions of electrons along with the high time resolution electric & magnetic data and ion distributions, to study in detail the structure of these FTEs.
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    ABSTRACT: Bright aurorae can be excited by the acceleration of electrons into the atmosphere in violation of ideal magnetohydrodynamics. Modeling studies predict that the accelerating electric potential consists of electric double layers at the boundaries of an acceleration region but observations suggest that particle acceleration occurs throughout this region. Using multispacecraft observations from Cluster, we have examined two upward current regions on 14 December 2009. Our observations show that the potential difference below C4 and C3 changed by up to 1.7 kV between their respective crossings, which were separated by 150 s. The field-aligned current density observed by C3 was also larger than that observed by C4. The potential drop above C3 and C4 was approximately the same in both crossings. Using a novel technique of quantitively comparing the electron spectra measured by Cluster 1 and 3, which were separated in altitude, we determine when these spacecraft made effectively magnetically conjugate observations, and we use these conjugate observations to determine the instantaneous distribution of the potential drop in the AAR. Our observations show that an average of 15% of the potential drop in the AAR was located between C1 at 6235 km and C3 at 4685 km altitude, with a maximum potential drop between the spacecraft of 500 V, and that the majority of the potential drop was below C3. Assuming a spatial invariance along the length of the upward current region, we discuss these observations in terms of temporal changes and the vertical structure of the electrostatic potential drop and in the context of existing models and previous single- and multispacecraft observations.
    Journal of Geophysical Research Atmospheres 12/2012; 117(A12):12203-. DOI:10.1029/2012JA017655 · 3.44 Impact Factor
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    ABSTRACT: A number of backscatter power enhancement events with “equatorward-moving radar auroral forms” in the high-latitude ionosphere were observed by SuperDARN CUTLASS Finland radar when the IMF was northward during 09:00–10:00 UT on 26 March 2004. These events were also associated with sunward flow enhancements at each location in the Northern Hemisphere which were shown in ionospheric convections measured by the SuperDARN radars. These are typical features of high-latitude (lobe) magnetic reconnections. The durations of the velocity enhancements imply that the evolution time of the lobe reconnections is about 8–16 min from their origin at the reconnection site to their addition to the magnetotail lobe again. In additional, the Double Star TC-1 spacecraft was moving from magnetosheath into magnetosphere, and crossing the magnetopause near the subsolar region during this interval, and observed typical low-latitude magnetic reconnection signatures. This infers that the dayside high- and low-latitude reconnections may occur simultaneously.
    Science China Technological Sciences 05/2012; 55(5). DOI:10.1007/s11431-012-4820-y · 1.11 Impact Factor
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    The Review of scientific instruments 05/2012; 83(5):059901. DOI:10.1063/1.4717726 · 1.58 Impact Factor
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    ABSTRACT: Quasi-static magnetic-field-aligned electric potential drops at altitudes between 1000 and 12000 km are able to accelerate charged particles into and out of the ionosphere above the aurora. Since 2008, Cluster has made regular passes through this so-called auroral acceleration region (AAR), facilitating studies of both the temporal evolution and spatial structure of these regions. Whilst the spacecraft can pass over this region with their foot-points separated by only fractions of a degree, this still translates to 10s km in the ionosphere, and this is comparable to the scale size of some auroral arcs. Consequently, the validity of assumptions made concerning magnetic conjugacy, or that the spacecraft are passing through the same acceleration region at different times, may be severely tested and must be closely examined. In this study, we examine a number of AAR crossings by the 4 Cluster spacecraft and compare the accelerated particle spectra recorded by the different spacecraft in order to determine the likelihood of their being conjugate or passing through the same feature at different times. From this, we attempt to understand the uncertainty in determining the temporal evolution and spatial structure of quasi-static potential drops in the AAR.
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    ABSTRACT: Advancement in solar-terrestrial science is more easily achieved through the use of multi-point and multi-mission measurement over single spacecraft methods. Multi-spacecraft observations can be used to address long standing questions regarding the connection between separated regions of the Earth's magnetosphere as well as help define observed phenomena with more clarity. One such long standing question relates to the relationship between widespread dipolarisation of the inner magnetosphere and the observable phenomenon of earthward fast flows in the tail plasma sheet (often termed "bursty bulk flows", or BBFs). The former is associated with the substorm expansion phase and the latter can be associated with reconnection at the near-Earth neutral line (NENL). We used all four Cluster spacecraft, in a multi-spacecraft configuration, to detect fast flows in the magnetotail plasma sheet region. The flows were measured using a multi-instrument approach, implementing ExB drift velocity data where 3D electric field data could be reliably reconstructed from the available 2D measurements, and particle instrument data at other times. Inter-calibration was performed using statistical methods. In addition to the Cluster fast flow detections, we used both Double Star spacecraft, when available, to detect reconfiguration of the magnetic field earthwards of the position of Cluster in the plasma sheet. Moreover, we made use of the Frey & Mende (2006) substorm onset list, compiled from data collected by the IMAGE mission, to relate observations of the tail to substorm phase. These multi-mission data were gathered from the 'tail season' intervals of 2004 & 2005. This multi-year period was chosen as they were the times when the conjunctions between Cluster & Double Star were favourable in the tail and the IMAGE mission was actively observing substorm onset signatures in Earth's auroral regions. We discuss our methods and report on progress.
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    ABSTRACT: We report our findings comparing the geometric factor (GF) as determined from simulations and laboratory measurements of the new Dual Electron Spectrometer (DES) being developed at NASA Goddard Space Flight Center as part of the Fast Plasma Investigation on NASA's Magnetospheric Multiscale mission. Particle simulations are increasingly playing an essential role in the design and calibration of electrostatic analyzers, facilitating the identification and mitigation of the many sources of systematic error present in laboratory calibration. While equations for laboratory measurement of the GF have been described in the literature, these are not directly applicable to simulation since the two are carried out under substantially different assumptions and conditions, making direct comparison very challenging. Starting from first principles, we derive generalized expressions for the determination of the GF in simulation and laboratory, and discuss how we have estimated errors in both cases. Finally, we apply these equations to the new DES instrument and show that the results agree within errors. Thus we show that the techniques presented here will produce consistent results between laboratory and simulation, and present the first description of the performance of the new DES instrument in the literature.
    The Review of scientific instruments 03/2012; 83(3):033303. DOI:10.1063/1.3687021 · 1.58 Impact Factor
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    ABSTRACT: Magnetic holes with relatively small scale sizes, detected by Cluster and TC-1 in the magnetotail plasma sheet, are studied in this paper. It is found that these magnetic holes are spatial structures and they are not magnetic depressions generated by the flapping movement of the magnetotail current sheet. Most of the magnetic holes (93%) were observed during intervals with Bz larger than Bx, i.e. they are more likely to occur in a dipolarized magnetic field topology. Our results also suggest that the occurrence of these magnetic holes might have a close relationship with the dipolarization process. The magnetic holes typically have a scale size comparable to the local proton Larmor radius and are accompanied by an electron energy flux enhancement at a 90° pitch angle, which is quite different from the previously observed isotropic electron distributions inside magnetic holes in the plasma sheet. It is also shown that most of the magnetic holes occur in marginally mirror-stable environments. Whether the plasma sheet magnetic holes are generated by the mirror instability related to ions or not, however, is unknown. Comparison of ratios, scale sizes and propagation direction of magnetic holes detected by Cluster and TC-1, suggests that magnetic holes observed in the vicinity of the TC-1 orbit (~7-12 RE) are likely to be further developed than those observed by Cluster (~7-18 RE).
    Annales Geophysicae 03/2012; 30(3):583-595. DOI:10.5194/angeo-30-583-2012 · 1.68 Impact Factor
  • Journal of Geophysical Research Atmospheres 01/2012; 117:A12203. · 3.44 Impact Factor
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    ABSTRACT: The dayside magnetopause is the primary site of energy transfer from the solar wind into the magnetosphere, and modulates the activity observed within the magnetosphere itself. Specific plasma processes operating on the magnetopause include magnetic reconnection, generation of boundary waves, propagation of pressure-pulse induced deformations of the boundary, formation of boundary layers and generation of Alfvén waves and field-aligned current systems connecting the boundary to the inner magnetosphere and ionosphere. However, many of the details of these processes are not fully understood. For example, magnetic reconnection occurs sporadically, producing flux transfer events, but how and where these arise, and their importance to the global dynamics of the magnetospheric system remain unresolved. Many of these phenomena involve propagation across the magnetopause surface. Measurements at widely-spaced (Δ ∼ 5 RE) intervals along the direction of dayside terrestrial field lines at the magnetopause would be decisive in resolving these issues. We describe a mission carrying a fields and plasmas payload (including magnetometer, ion and electron spectrometer and energetic particle telescopes) on three identical spacecraft in synchronized orbits. These provide the needed separations, with each spacecraft skimming the dayside magnetopause and continuously sampling this boundary for many hours. The orbits are phased such that (i) all three spacecraft maintain common longitude and thus sample along the same magnetopause field line; (ii) the three spacecraft reach local midday when northern European ground-based facilities also lie near local midday, enabling simultaneous sampling of magnetopause field lines and their footprints. KeywordsMagnetopause–Magnetic reconnection–Solar wind–magnetosphere coupling–Cosmic vision
    Experimental Astronomy 01/2012; 33(2-3):1-37. DOI:10.1007/s10686-011-9245-2 · 2.66 Impact Factor
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    ABSTRACT: The temporal sequence of events at substorm onset requires the generation and propagation of electromagnetic waves as the system evolves from its pre- to post-onset state. Such waves offer a unique diagnostic for the dynamics of this system, and the important coupling between the equatorial magnetosphere and auroral onset dynamics in the ionosphere. We detail the ground auroral and magnetic response to both substorms and nightside auroral activations, with particular focus on characterising the space-based counterparts of the well-known onset sequence of the auroral substorm in the ionosphere. We present specific case studies and some statistics of the evolution of magnetic wave power and auroral intensity throughout the late growth phase and expansion phase of the substorm cycle. We show strong evidence that some substorm-related auroral enhancements are clearly and demonstrably linked to the evolution of ULF wave power, such as onset arc auroral beads, whilst others have a smaller ULF wave signature. We utilise multi-point multi-instrument conjunctions from THEMIS, Cluster and GOES and other platforms to probe the magnetospheric counterpart of the ground-based signatures through onset, using a range of time-series analysis techniques. In particular, we examine the implied mapping of Pi1-2 onset signatures, which often show a clear ionospheric epicentre, to dynamical processes in the magnetotail in an attempt to diagnose the plasma physics linking onset arcs to their energy sources in the tail.
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    ABSTRACT: We present the results of a survey of Cluster PEACE and CIS-CODIF data taken in the 2001-2006 tail seasons, building on the work of Walsh et al. (GRL, 2011). We examine the average pitch angle distributions of protons and electrons in the magnetotail as a function of proton plasma beta, restricted to times when the magnetosphere was exposed to steady (on a 3 hour timescale) IMF conditions and focussing in particular on dawn-dusk asymmetries. We confirm that, on average, the 2 component proton plasma sheet exists duskward of the noon-midnight meridian under steady northward IMF. An associated population of cold electrons is also observed. Dawnward of the noon-midnight meridian there are no significant fluxes of the cold component of protons and much reduced fluxes of the cold electron component, implying transport across the dusk magnetopause is the dominant formation mechanism of the two component plasma sheet for both protons and electrons. Under southward IMF, dawn-dusk asymmetries in the protons are controlled by the Y component of the IMF. For the electrons higher fluxes of high energy, field-aligned, particles are observed at dusk than at dawn. This suggests a link to a duskward offset of the tail neutral line and the preferential observation of substorm-related tail signatures in the premidnight sector. We also consider the relationship between the observed particle populations and the average behaviour of the large-scale magnetotail current systems as revealed by the Curlometer.